The ever increasing need for more frequency spectrum in wireless communications has recently shifted the attention of standards developers to unlicensed frequency bands. Given the large amount of spectrum available in unlicensed frequency bands, the capacity in Long-Term Evolution (LTE), developed for the 3rd Generation Partnership Project (3GPP), could potentially be increased. However, unlicensed frequency bands come with additional challenges associated with, e.g., managing interference and coexisting with other technologies.
In response, a Licensed-Assisted Access (LAA) framework in 3GPP has been introduced. The LAA framework (3GPP Release 13) builds on carrier aggregation solutions introduced in Release 10 LTE to access the additional bandwidth in the unlicensed frequency bands. FIG. 1 shows an exemplary LAA framework 40, where an enhanced NodeB (eNB) uses and configures a secondary cell (SCell) 44 or an LAA carrier on the unlicensed band. The primary cell (PCell) 42 carries the more critical real-time traffic and control information, while the LAA carrier will be used to increase the capacity for less sensitive data, e.g., best effort data. The next 3GPP release may enable LAA to operate without a licensed primary carrier in a more stand-alone fashion, which may present additional challenges. In contrast with legacy Wi-Fi standards, the LAA uplink is scheduled and is not allowed to contend for access on its own. Instead, the User Equipments (UEs) receive so called uplink grants from the eNB containing information regarding when the UEs are allowed to transmit. Uplink and downlink transmissions are usually contained within the same transmission opportunity (TXOP). This way, the eNB protects the uplink from channel competing technologies.
The need to coexist with other technologies operating in the same frequency band presents a challenge to the use of the unlicensed frequency bands for the LAA carrier. The major technology operating in the unlicensed frequency band today is Wi-Fi, which is governed by the Wi-Fi standard IEEE 802.11 and all its variants. The traditional method used by IEEE 802.11 devices to coexist and share the spectrum is the so called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Devices following this scheme use carrier sensing to detect other transmissions, and perform back-off to defer transmissions until the channel is found idle. This algorithm is also known as a Listen-Before-Talk (LBT) scheme.
IEEE 802.11ax is the latest Wi-Fi technology variant and is currently being standardized by the IEEE 802.11ax task group (TGax). Two of the most prominent new features in 802.11ax are uplink/downlink Orthogonal Frequency Division Multiple Access (OFDMA) and uplink Multi-User Multiple-Input-Multiple-Output (MU-MIMO). Another important feature, which is generally not found in earlier variants of IEEE 802.11, is a scheduled uplink. Traditionally the uplink and downlink in Wi-Fi have operated under the same channel access rules. Specifically, the non-network nodes have contended for the channel using CSMA/CA. The motivation behind this design has been to maintain a low complexity and has not caused any problems for smaller networks. However, when the number of uplink devices increases, as expected with 802.11ax, the downlink will gradually receive a smaller share of the capacity, i.e., 1/(N+1) of the total capacity when there are N uplink devices and assuming all devices have non-empty buffers.
In 802.11ax, a tone plan has been set for a new Fast Fourier Transform (FFT) size of 256 (4 times the size used by the legacy standard, which is 64). The smallest allocated sub-band, generally referred to as a resource unit (RU), consists of 26 subcarriers, where each RU contains two pilot tones. The largest RU for 20 MHz contains 242 tones, including 8 pilot tones. There are many more tone unit sizes for different bandwidths. This tone plan is required for resource allocation with OFDMA in uplink and downlink. The increased FFT size in 802.11ax results in OFDM symbols that are four times longer for the data field. Note that the legacy preamble in 802.11ax packets still uses the legacy FFT size of 64.
In conventional LAA systems, the eNB contends for access for both the downlink (DL) and the uplink (UL). In a network with contending Wi-Fi devices (one BSS), e.g., devices bound by the IEEE 802.11 standards, the LAA cell will at most receive 1/(N+2) of the total capacity for both the uplink and downlink, when all devices have data to transmit, due to the fact that the network node as well as the Wi-Fi devices are contending for the channel. The coexistence between two LAA cells will give a fair sharing of the capacity between the cells, in contrast to a scenario where a LAA cell is coexisting with one Wi-Fi BSS. Thus, there is a need for additional channel allocation solutions. In particular, there is a need for solutions that enable the LAA to share the radio resources in a more even way with Wi-Fi, while still complying with all rules defining channel access fairness.